The use of manganese dioxide as an active cathode material in dry cells is well known. Among the forms of manganese dioxide which have been found to exhibit desirable electrochemical properties in both aqueous and nonaqueous cells is electrolytic manganese dioxide ("EMD"). As is noted in U.S. Pat. No. 4,048,027, in the prior art the production of electrolytic manganese dioxide has typically entailed the feeding of a preheated MnSO.sub.4 --H.sub.2 SO.sub.4 bath into an electrolytic cell which is operated with direct current under the following general conditions:
a. electrolytic bath concentration--MnSO.sub.4 : 0.5 to 1.2 mole/liter; H.sub.2 SO.sub.4 : 0.5 to 1.0 mole/liter; PA0 b. electrolytic bath temperature, 80.degree. C. to 100.degree. C.; and PA0 c. an anodic current density of 7 to 12 mA/cm.sup.2 (i.e., about 7.6 to 13.1 A/ft.sup.2). PA0 (1) preparing a bath comprising (a) between about 0.5 and about 1.3 moles/liter of an aqueous solution of at least one manganese salt selected from the group consisting of manganous sulfate, manganous chloride and manganous nitrate, (b) a sufficient quantity of at least one acid selected from the group consisting of sulfuric acid, nitric acid and hydrochloric acid such that the pH of the bath is between about 1 and about 2, and (c) at least one suspension agent selected from the group consisting of insoluble multivalent elements and of insoluble compounds of multivalent metals other than manganese; PA0 (2) heating said bath to between about 90.degree. C. and about 100.degree. C.; and PA0 (3) electrolyzing the heated bath in an electrolytic cell having at least one anode and at least one cathode, preferably at an anode current density of between about 10 and about 20 amperes per square foot (i.e., between about 10.8 and about 21.5 mA/cm.sup.2).
The anode employed in such electrolytic process is generally composed of titanium, lead alloy or carbon. The cathode is typically composed of carbon, lead or platinum. During electrolysis manganese dioxide is deposited at the anode. The electrolytic manganese dioxide is removed from the anode and after conventional post-treatment, it is ready for use as an active cathode material in dry cells.
In conventional prior art processes, the current density is the principal factor with respect to productivity. However, increasing the current density in excess of the range recited above in order to increase productivity leads to oxygen evolution at the anode with consequent lowering of EMD production efficiency. Such oxygen evolution may result in passivation and/or consumption of the anode. Further, the EMD produced by electrolysis at these higher current densities exhibits undesirable physical properties.
In order to overcome such limitation upon productivity, it has become known in the art that the current density of an EMD production bath may be increased without an adverse effect on voltage if a suspension of manganese oxide particles is employed in the bath. Thus, U.S. Pat. No. 4,405,419 discloses a process for producing manganese dioxide comprising the electrolysis of a slurry of particles of manganese oxides in an aqueous solution of manganese sulfate at current densities of up to 16 mA/cm.sup.2 (i.e. about 14.9 A/ft.sup.2). Similarly, Japanese Patent Disclosure No. 1976-104499 discloses a process for the production of EMD by the electrolysis of a solution of manganese sulfate containing sulfuric acid in the presence of a suspension of particles of manganese oxide other than divalent manganese employing anodic current densities of up to 20 mA/cm.sup.2 (18.6 A/ft.sup.2).
The manganous salts, derived from manganous ores, that are typically employed as starting materials in the EMD production process generally contain some amount of undesirable impurities, particularly potassium. During the electrolysis of these salts potassium is incorporated into the resultant EMD. As is well known in the art, the presence of potassium in electrolytic manganese dioxide is undesirable, having adverse effects on the utilization of such manganese dioxide in alkaline cells. It would therefore be beneficial to provide a process for the manufacture of EMD, which process would reduce the amount of potassium incorporated into the product EMD from the manganese ore starting material.
It is therefore an object of this invention to provide a bath for the production of EMD which bath will allow an increased production rate of EMD.
It is a further object of the invention to provide a bath for the production of EMD which bath when electrolyzed will produce EMD which performs desirably when employed in acidic, alkaline and nonaqueous battery systems.
It is another object of this invention to provide a bath for the production of EMD, which bath when electrolyzed will produce EMD having a reduced potassium content.
It is yet another object of this invention to provide a process for the production of EMD, which process will permit rapid production of EMD.
The above and additional objects will become clear from the following description and Examples.